Process for producing activated carbon material, and electric double layer capacitor employing the same

ABSTRACT

An activated carbon for an electric double layer capacitor electrode, which comprises a stacking structure having 2 layers or less of in a proportion stacking of from 25 to 80% and a stacking structure having 5 layers or more in a proportion of from 2 to 30% in the distribution of a stacking structure as obtained by analysis of the X-ray diffraction pattern of (002) plane, and which has a specific surface area of from 500 to 2,800 m 2 /g and a total pore volume of from 0.5 to 1.8 cm 3 /g.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an activated carbon useful for anelectrode as, for example, an electric double layer capacitor, a processfor producing the activated carbon and an electric double layercapacitor employing the activated carbon. The electric double layercapacitor of the present invention can be widely used for power sourcesfor various portable apparatus, standby power sources for domesticelectrical equipment, UPS for optical communication, power sources forelectric automobiles and the like.

2. Discussion of Background

As an electric double layer capacitor, a coin type obtained in such amanner that an element having a pair of electrodes consisting mainly ofan activated carbon formed on a current collector and a separatorsandwiched therebetween, together with an electrolytic solution, issealed in a metal casing by means of a metal lid and a gasket insulatingthe casing from the lid, and a wound type obtained in such a manner thata pair of sheet electrodes is wound by means of a separator interposedtherebetween to obtain a wound element, which is accommodated in a metalcasing together with an electrolytic solution, and sealed in the casingso that the electrolytic solution does not evaporate from an opening ofthe casing, have been known.

Further, for an application which requires a large current and largecapacitance, stack type electric double layer capacitor having anelement obtained by stacking a large number of sheet electrodes by meansof a separator interposed therebetween, incorporated therein, has beenproposed (JP-A-4-154106, JP-A-3-203311, JP-A-4-296108). Namely, aplurality of sheet electrodes formed into a rectangle as positiveelectrodes and negative electrodes are alternately stacked one onanother by means of a separator interposed therebetween to obtain astacked element, a positive electrode lead material and a negativeelectrode lead material are connected with the respective terminals ofthe positive electrodes and the negative electrodes by caulking, and theelement in such a state is accommodated in a casing, impregnated with anelectrolytic solution and sealed with a lid.

Heretofore, as an electrolytic solution for an electric double layercapacitor, a solvent having a high dielectric constant such as water orpropylene carbonate has been used so as to dissolve an electrolyte at ahigh concentration. As an electrode for an electric double layercapacitor, wherein the charge of an electric double layer formed on thesurface of the activated carbon itself contributes to the capacitance ofthe electric double layer capacitor, made mainly of an activated carbonhaving a large specific surface area has been employed.

An activated carbon is produced usually by carbonizing and activating acarbon source derived from a plant such as a sawdust or a coconut shell,a carbon source derived from a coal/petroleum material such as a coke ora pitch, or a synthetic high polymer carbon source such as a phenolresin, a furfuryl alcohol resin or a vinyl chloride resin.

The carbonization means a sequence of processes, in which an organiccarbon material as the above carbon source is heated and changed so thatthe carbon is concentrated through a bond-cleavage, a decomposition anda polycondensation to be converted to a solid carbon product (acarbonized product). The carbonization is carried out usually by heatinga carbon source in a non-oxidizing atmosphere at a temperature of from300° C. to 2,000° C.

The activation is a process in which a precise porous structure isformed in the carbonized product obtained above. Usually, the activationis carried out by a gas activation process in which the carbonizedproduct is heated in a weak oxidizing gas containing carbon dioxide orwater vapor at a temperature of from 500° C. to 1,100° C., so that thecarbonized product is oxidized and exhausted to have a porous structureand to increase its surface area. Otherwise, the activation is carriedout by a chemical activation process (an alkali activation process) inwhich the carbonized product is mixed with an alkali metal hydroxide(such as KOH) in an amount of equal to or several times the mass of thecarbonized product, and then the mixture is heated at a temperature offrom the melting point of the metal hydroxide to 1,000° C., preferablyfrom 400 to 800° C. in an inert atmosphere for from several tens minutesto 5 hours so that the carbonized product can have a porous structureand increase its surface area. The alkali metal hydroxide is removed byadequate washing after the activation process is finished.

As important properties required for an electric double layer capacitor,a) large capacitance, b) a high energy density, c) high durability whencharging and discharging cycles are repeated, and d) a low internalresistance, may, for example, be mentioned.

As an electrode material presenting large capacitance among theproperties, an activated carbon obtained by activating a carbon materialderived from a pitch by heating in the coexistence of an alkali metalhydroxide (an alkali activation) has been proposed. (JP-A-5-258996,JP-A-10-199767, U.S. Pat. Nos. 3,817,874, 4,082,694).

Further, it is reported that an activated carbon obtained by alkaliactivation of a carbon material having a relatively developedgraphitizability, such as a pitch showing optical anisotropy i.e.so-called mesophase pitch as a carbon source, has large capacitance permass, and has a relatively high bulk density. Accordingly, when theactivated carbon is formed into an electrode, the electrode has a highdensity, whereby an electric double layer capacitor having the electrodehas large capacitance per unit volume (JP-A-2-185008, JP-A-10-121336).

However, according to the studies of the present inventors, theelectrode containing an activated carbon derived from the material ofwhich the graphitizability is relatively developed has problems: theelectrode incorporated into a capacitor cell case tends to expandremarkably at the process of charging, resulting in the failure of thecell case, moreover due to the expansion of the electrode thecapacitance per volume of the electrode after expansion is not largeenough comparing to that calculated from the volume of the electrodebefore expansion.

On the other hand, in an electric double layer capacitor, it is alsoknown to use an electrode containing an activated carbon which has alarge specific surface area, obtained by steam activation or alkaliactivation of a carbon source constituted by a carbon material having arelatively low graphitizability, such as a thermosetting resin forexample a phenol resin or a pitch showing optical isotropy. This type ofthe capacitor has a large capacitance per unit mass, a low expansion ofthe electrode at the time of charging and a high durability whencharging and discharging are repeated for a long period of time.However, this type of the capacitor has a problem that the capacitanceper unit volume tends to be small because a bulk density of theactivated carbon is low.

Although it has not been fully explained why an activated carbonelectrode obtained by alkali activation of a carbon material having arelatively developed graphitizability expands during charging, thepresent inventors consider its reason as follows: In general, it tendsto be difficult to increase the specific surface area of a carbonmaterial having a developed graphitizability by gas activation.Accordingly, an alkali activation is employed to increase the specificsurface area of the carbon material. The mechanism of the activation byan alkali metal hydroxide is not clearly understood in detail for themost part. However. for example using the example of an activation byKOH, KOH infiltrates into between carbon layers at a relatively lowtemperature of from 400 to 500° C., during this step, carbonation ofcarbon and gasification of carbon with generated water or carbonicdioxide gas are caused, whereby carbon is consumed to increase thespecific surface area, and a potassium metal generated by reduction ofKOH is intercalated as a guest into between carbon layers (a host) (NewEdition Activated Carbon, Yuzo Sanada, Kodansha Ltd. Publishers,Scientific).

It is estimated that in an activated carbon subjected to alkaliactivation has weak bonding force between carbon layers caused by theintercalation mentioned above. Accordingly, it is considered that whenthe activated carbon is used as an electrode for an electric doublelayer capacitor, not only ions are adsorbed into pores in the activatedcarbon but some ions are adsorbed into pores to widen the space betweencarbon layers at the time of charging.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the above problemsof prior art and to provide an activated carbon having large capacitanceper unit volume when it is used as an electrode for an electric doublelayer capacitor, and a process for producing the same. Further, thepresent invention is to provide an electric double layer capacitorhaving large capacitance and high reliability wherein the expansion ofan electrode is suppressed using the activated carbon as an electrodematerial.

As a result of carrying out extensive studies to solve the aboveproblems of the conventional activated carbon, the present inventorshave found the following facts: By mixing a carbon source to form highgraphitizability carbon and a carbon source to form highnon-graphitizability carbon, and, followed by subjecting the resultingmixture to a carbonization and an activation treatment can be obtainedan activated carbon. Thus obtained activated carbon has an advantage oflarge capacitance per unit volume caused from large capacitance and alarge bulk density of the activated carbon derived from a highgraphitizability carbon component, and also an advantage of largecapacitance per unit mass, a low deterioration in the performance afterthe repeating of charging and discharging cycle, and a low expansion ofan activated carbon particle by ion absorption caused from an activatedcarbon derived from a high non-graphitizability carbon component. At thesame time the above activated carbon further has property of complementto the weaknesses of the activated carbon derived from the otherrespective carbon sources.

According to the present invention, there is provided an activatedcarbon for an electric double layer capacitor electrode, which comprisesa stacking structure having 2 layers or less in a proportion of from 25to 80% and a stacking structure having 5 layers or more in a proportionof from 2 to 30% in the distribution of a stacking structure as obtainedby analysis of the X-ray diffraction pattern of (002) plane, and whichhas a specific surface area of from 500 to 2,800 m²/g and a total porevolume of from 0.5 to 1.8 cm³/g.

According to the present invention, there is provided a process forproducing an activated carbon for an electric double layer capacitorelectrode, which comprises: (1) a step of mixing a carbon source whichforms a non-graphitizable carbon by heating (hereinafter also referredto as a non-graphitizable carbon) and a carbon source which forms agraphitizable carbon by heating (hereinafter also referred to as agraphitizable carbon); (2) a step of carbonizing the resulting mixtureby means of heating; and (3) a step of activating the carbonizedproduct.

Further, according to the present invention, there is provided anelectric double layer capacitor comprising an electrode containing theactivated carbon described above.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The fundamental feature of the activated carbon according to the presentinvention is that the carbon forming the skeleton of the activatedcarbon comprises a high graphitizability carbon component and a highnon-graphitizability carbon component, both of which are homogeneouslymixed.

Herein, the high graphitizability carbon component has a stackingstructure with a relatively high regularity of six-membered ringcondensed layers so called “turbostratic structure”, wherein layeredsurfaces are firstly superimposed in a parallel and equal distancesimilar to a graphite, and secondary, are not completely oriented andoverlapped irregularly, although that is not a single complete crystalstructure of a graphite.

On the other hand, the high non-graphitizability carbon componentcomprises space lattice wherein carbon six-membered rings areinter-connected irregularly, and has almost no stacking of six-memberedring condensed layers called amorphous carbon or a lamination ofsix-membered ring condensed layers having structure poor in theregularity.

These different structures can be identified qualitatively bydiffraction peaks from (002) of carbon in a wide-angle X-ray diffractionas follows: the diffraction peaks of a high graphitizability part arerelatively sharp, and the diffraction peaks of a highnon-graphitizability part are wide. When carbon having differentstructure is homogeneously mixed, the overlapped diffraction peaksderived from the above diffraction peaks are observed to meancoexistence of both structures. More quantitatively, it can be made byanalyzing diffraction peaks from (002) of carbon in a wide-angle X-raydiffraction in the following method (Nemoto et al. “Research Report of aFaculty of Hokkaido University, volume 91, (1978)).

Namely, the diffraction intensity I₀₀₂ of (002) band of carbon by X-raydiffraction is corrected by an atomic scattering factor f andtransformed by the Fourier transform to obtain a patterson function P(u)represented by the formula (1):

P(u)=2∫₀ ^(∞)(I ₀₀₂ /f ²)cos 2πsuds  (1)

(in the formula, I₀₀₂ represents strength of (002) band, f represents anatomic scattering factor, s represents a distance of reciprocal latticeand u represents a distance of real space.)

This patterson function P(u) represents the number of the carbon layersurface which is recognized at a vertical distance from the certaincarbon layer surface. The ratio of the stacking structure containing Nlayers in all of stacking layers can be obtained from the peak areawhich P(u) accounts for on the background. Thus, in the above analysis,a large high stacking structure ratio represents high carbongraphitizability, and a large low stacking structure ratio representsrelatively high carbon non-graphitizability.

As a result of the studies by the inventors of the present inventionfrom the above viewpoint, the following facts have been found: a highstacking structure of from 5 layers to 6 layers is observed in acarbonized product powder derived from e.g. a petroleum pitch which is agraphitizable carbon source. On the other hand, a stacking structure of3 layers or more does not exists in a carbonized product powder derivedfrom a phenol resin such as a novolac resin which is a non-graphitizablecarbon source. Thus, it preferred that a stacking structure of 5 layersor more is used for a measure of high graphitizability carbon, and astacking structure of 2 layers or less is used for a measure ofnon-graphitizability carbon.

Table 1 shows one example of the results obtained by calculating theamount of the stacking structure containing the layer surface of N layeras obtained by analyzing, according to the method mentioned above, thediffraction peek of (002) plane of the carbonized product, which wasprovided by carbonizing of the mixture of phenol resin and a coal tarpitch at 700° C. as raw material, herein the mixing ratio was determinedby considering a carbonization yield of each raw materials.

TABLE 1 Carbon derived from Resin % 2 Layer 3 Layer 4 Layer 5 Layer 6Layer 0 0.19 0.28 0.25 0.15 0.12 10 0.25 0.25 0.23 0.14 0.11 20 0.320.22 0.20 0.12 0.09 30 0.39 0.20 0.18 0.11 0.08 40 0.46 0.17 0.15 0.090.07 50 0.52 0.14 0.13 0.08 0.06 60 0.59 0.11 0.10 0.06 0.05 70 0.660.08 0.08 0.05 0.04 80 0.73 0.06 0.05 0.03 0.02 90 0.79 0.03 0.03 0.020.01 100 0.86 0.00 0.00 0.00 0.00

From the summarized results of the above studies, the activated carbonof the present invention comprises a stacking structure having 2 layersor less in a proportion of from 25 to 80%, preferably from 30 to 75%,particularly preferably from 40 to 65%, and a stacking structure having5 layers or more in a proportion of from 2 to 30%, preferably from 5 to25%, particularly preferably from 10 to 20%, in the distribution ofstacking structures containing layer surface of N layer as obtained fromthe analysis of the X-ray diffraction pattern of (002) band.

The activated carbon of the present invention comprises a homogeneouslymixed structure of a high graphitizability carbon part and a highnon-graphitizability carbon part, wherein the above carbon parts havingdifferent graphitizability exist presumably in the completely andstrongly interconnected state in a particle of the activated carbon.

The reason why excellent properties can be attained in the case of usingthe activated carbon having the above-mentioned structure as anelectrode material for an electric double layer capacitor, is estimatedthat the skeleton of the high non-graphitizability carbon restrict theskeleton of the high graphitizability carbon, which will be explained inthe following.

The activated carbon made from high non-graphitizability carbon has alow bulk density because of the low graphitizability of carbon and thedisordered structure, whereby it is difficult to fill many electrodesmade of an activated carbon into unit volume. As a result, sufficientlyhigh capacitance per unit volume, which is important characteristics foran electric double layer capacitor cannot he obtained. However, theactivated carbon made from high non-graphitizability carbon has highcapacitance per unit mass because a high specific surface area is easilyobtained by activation. The activated carbon made from highnon-graphitizability carbon further has skeleton structure made of spacelattice containing an irregular inter connection of six-membered ring,that is, the skeleton structure of the activated carbon has largestrength, whereby low changes in the volume of the activated carbonparticle at the adsorption-desorption of ions can be attained.

On the other hand, the activated carbon made from high graphitizabilitycarbon has relatively high capacitance per unit mass and a high bulkdensity from its structural reason, whereby it is possible to fill manyactivated carbon electrodes into unit volume. As a result, sufficientlyapparent high capacitance per unit volume, which is importantcharacteristics for an electric double layer capacitor can be obtained.However, as already mentioned, the activated carbon particle (electrode)made from high graphitizability carbon has high volume expansion at thetime of charging, whereby capacitance per unit volume correctlycalculated from the volume after electrode expansion is not sufficientlyhigh. Particularly, there is a problem that the destruction of the outercasing of the capacitor caused by the expansion of the electrode islikely to occur since the electrode is heavily expanded at the time ofcharging.

The activated carbon of the present invention has a homogeneously mixedstructure of a high graphitizability part and a highnon-graphitizability part, wherein a stacking structure having 2 layersor less is at least from 25 to 80% and a stacking structure having 5layers or more is at least from 2 to 30% in the distribution of stackingstructures containing layer surface of N layer as obtained by theanalysis of the X-ray diffraction pattern of (002) band. Accordingly,the skeleton of a high non-graphitizability activated carbon havingsmall volume change at the time of charging and discharging caused bylarge structural strength restricts the skeleton of a highgraphitizability activated carbon and suppress the volume expansion of ahigh graphitizability activated carbon which has apparent highcapacitance per unit volume but large volume expansion after charging.Consequently, as the advantages of both activated carbon are combined,it is possible to form activated carbon material for an electric doublelayer capacitor electrode having large capacitance per unit volume andexcellent long reliability as well as restricted volume change of theactivated carbon particles at the time of charging and discharging.

When the proportion of the stacking structure having 2 layers or less isless than 25%, the effect of restricting the volume expansion at thetime of charging will be low. When such a proportion exceeds 80%, thecapacitance will not become high enough because of too much highnon-graphitizability carbon part. On the other hand, when the proportionof the stacking structure having 5 layers or more is less than 2%, thesame problem caused by too much non-graphitizability carbon part willoccur. When such a proportion exceeds 30%, the volume expansion at thetime of charging will become large because of too much highgraphitizability carbon part.

In addition to the above structure, the activated carbon of the presentinvention for the electrode has a specific surface area of from 500 to2,800 m²/g, preferably from 1,300 to 2,500 m²/g and a total pore volumeof from 0.5 to 1.8 cm³/g, preferably from 0.7 to 1.5 cm³/g. When thesevalues are less than the above range, sufficient capacitance will not beobtained. When these values are larger the above range, the activatedcarbon will become bulky, whereby the capacitance per unit volume of theelectrode will be deteriorated.

The excellent characteristics of the activated carbon of the presentinvention can be attained by the homogeneous mixture of a highgraphitizability structure part and a high non-graphitizabilitystructure part and by strong connection between them, while the skeletonstructure of the activated carbon is restricted. As shown from Examplesand comparative Examples later described, the excellent propertiesobtained by the activated carbon of the present invention cannot beattained, when activated carbon particles solely made of a highgraphitizability structure part and activated carbon particles solelymade of a high non-graphitizability structure part are simply mixed.

The activated carbon of the present invention comprising a highgraphitizability part and a high non-graphitizability part is obtainedby the following process: (1) a step of homogeneously mixing a carbonsource which forms a non-graphitizable carbon by heating and a carbonsource which forms a graphitizable carbon by heating; (2) a step ofcarbonizing the resulting mixture by means of heating; and (3) a step ofactivating the resulting carbonized product.

An non-graphitizable carbon (hard carbon) means carbon which is noteasily converted to graphite by a graphitizing treatment. As alreadymentioned, this carbon has a structure in which six-membered rings areirregularly interconnected so that the layer surfaces is not likely toconnect with each other without large atom migration, and itscrystallization will not easily proceed.

As a carbon source to form such non-graphitizable carbon can be usedgenerally a thermosetting resin such as a phenol resin, a melamineresin, a urea resin, a furan resin, an epoxy resin, an alkyd resin, anunsaturated polyester resin, a diallylphthalate resin, a furfural resin,a silicone resin, a xylene resin, an urethane resin, etc. A phenol resinis preferred from the points of easy handling at the production, highyield of carbonization, large solubility to a solvent, large mutualsolubility with graphitizable carbon described later, etc.

When a phenol resin is employed as a thermosetting resin, a resol typephenol resin and/or a novolac type phenol resin may be preferably used.

A graphitizable carbon (soft carbon) means a carbon which can beconverted to graphite by a graphitizing treatment. The crystallite ofthis carbon has a disordered structure similar to graphite, wherein thelayer surfaces are superimposed in parallel and equal distance andranged in the same direction so that the layer surfaces will disappearwith small atom migration to become graphite.

As a graphitizable carbon source which forms such a graphitizable carbonby heating at a high temperature, a common thermoplastic resin such as avinyl chloride type resin, polyacrylonitrile, polybutyral, polyacetal,polyethylene, polycarbonate or a polyvinyl acetate may, for example, bementioned. A pitch type material such as a petroleum type pitch or acoal type pitch or coke obtained by subjecting those types of pitch to aheat treatment may also be mentioned. A condensed polycyclic typehydrocarbon compound such as naphthalene, phenanthrene, anthracene,triphenyrene and pyrene may also be employed. A fused heterocycle typecompound such as indole, quinoline, carbazole and acridine may also beemployed. Among them, a pitch type material is preferably used from thepoints of low cost, a high yield of carbonization, its easy conversionto a liquid state by heating, large solubility to a solvent, etc.

(1) First, in the step of mixing, a carbon source which forms anon-graphitizable carbon by heating and a carbon source which forms agraphitizable carbon by heating are mixed homogeneously.

The mixing ratio of the carbon source which forms non-graphitizablecarbon by heating/the carbon source which forms graphitizable carbon byheating is preferably within the range of front 9.5/0.5 to 1/9 in themass ratio at the time of mixing of the respective carbon sources,although the mixing ratio varies depending on the residual amount of thecarbon after carbonization of the respective carbon sources.

In general, the carbon source which forms non-graphitizable carbon byheating and the carbon source which forms graphitizable carbon byheating may be homogeneously mixed in the powdered form of respectivecarbon sources. Further, in order to enhance the homogeneousness ofmixing, it is preferred that at least one of the carbon sources in aliquid state and the other carbon source in the form of fine powder aremixed in a solid-liquid type mixing. More preferably, both carbonsources are mixed in their liquid state for homogeneous mixing.

When the carbon sources are mixed in their powdered form, the powders ofthe carbon sources preferably have a particle diameter of 100 μm orless, more preferably from 0.5 to 20 μm, because the finer carbonparticle of the carbon sources, the more homogeneous mixing is expectedsince the particles to be mixed can be close to each other. As a mixer,a usual mixer for solid material such as a V type mixer, a horizontalcylindrical type mixer, a ribbon type mixer, a vertical screw typemixer, a Mueller type mixer, a reverse rotation Mueller type mixer, etc.may be employed.

In order to obtain the fine powders having a particle diameter mentionedabove, the following grinding machine may be appropriately employeddepending on the properties of the original carbon sources and theobjective particle diameter of the fine powder: a jaw crusher, ajairetry crusher, a single roll crusher, a cone crusher, a dodgecrusher, a double-roll crusher, an edge runner, a hammer mill, a rotarycrusher, an impeller breaker, a ball mill, a conical mill, a tube mill,a rod mill, an attrition mill, a hammer mill, a jet mill, or a micronmill.

When it is difficult to obtain the fine powders having the aboveparticle diameter beforehand, the respective carbon sources which havebeen roughly crushed into around several millimeters may be mixed whilethey are ground using a dry-type ball mill, a wet-type ball mill, anedge runner, a colloidal mill, etc.

The powders homogeneously mixed on the order of micrometer as describedabove are decomposed and polycondensated to concentrate the carbon inthe step of carbonization, while each mixed state is maintained. Thus,it is considered that the respective parts derived from the carbonsources (a high graphitizability part and a low graphitizability part)may be homogeneously mixed and a solid carbon product having a structurewherein both type carbons are strongly connected with each other can beobtained.

When one or both of the carbon sources is mixed in its liquid state, itis not preferred that such a liquid has large viscosity. The aboveliquid is preferably a liquid having a viscosity to the extent that theliquid can be stirred and mixed using a mixer such as a stirrer and akneader. The viscosity of the liquid at a temperature under which atleast one of liquid carbon sources is mixed preferably at level of20,000 poise or less.

While several methods for producing liquid state of the carbon sourcemay be conceivable, the most simple one is to use a carbon source whichis liquid at room temperature because of low molecular weight or acarbon source which can be melted to become liquid by heating it at atemperature of not more than several hundred ° C. Another method forproducing liquid state of the carbon is to dissolve the carbon sourcematerial into an appropriate solvent. When the carbon sources is mixedin the liquid state in which one or both of the carbon sources is meltedby e.g. heating, or is dissolved in e.g. an organic solvent, both carbonsources can be mixed more homogeneously, compared with when the carbonsources are mixed in a solid—solid form. When both carbon sources aremixed in their liquid states, most homogeneous mixing can be attained.

Further, when at least one of the carbon sources is in a liquid state,micro void in the carbon sources is filled with such a liquid, therebyincreasing bulk density of the activated carbon finally obtained throughthe carbonizing step and the activation step described later. Thefilling of the micro void with the liquid is not necessary to be carriedout in the step of mixing, because similar effects can be attained ifsuch filling is carried out at the step of curing or the step ofcarbonization. Thus, from this reason, at least one of the carbonsources is preferred to be in liquid state from the step of mixing tothe step of carbonization.

When either of the carbon sources is not malted by heating, for example,when one of the carbon sources is a thermosetting resin, it is preferredthat the other carbon source is melted by heating, or dissolved into anorganic solvent. The obtained liquid may be added to the carbon sourcein the fine powder form, and then mixed using a mixer capable of asolid-liquid mixing, such as a paddle type stirrer, a turbine typestirrer, a ball mill, an edge runner, etc, in this case, homogeneous andprecise mixing can be attained as compared with the case where bothcarbon sources are mixed in their powdered forms. In this occasion, whenthe viscosity of the mixture is high because of the insufficient themelted component, it is preferred to use a mixer capable of mixing whileimparting strong shear force to the mixture, such as a kneader mixer, ainternal mixer, a roll mixer, etc.

Further, in order to mix the carbon sources which are not of a highmolecular material in more completely, one of the carbon sources may beadded to the other carbon source in its polymerization step so that arandom, a block or a graft polymerization is carried out.

(2) Next, the step of carbonization is carried out wherein the mixtureof a non-graphitizable carbon source and a graphitizable carbon sourceobtained above is carbonized by heating.

The carbonization of the mixture in the present invention is carried outby so called a solid phase carbonization reaction. Accordingly, when themixture is in a liquid state, it is preferably subjected to a curetreatment before the carbonization wherein the mixture is heated at thetemperature of around from 150 to 350° C. in the presence or in theabsence of an appropriate catalyst in an inactive atmosphere or in anoxidizing atmosphere so as to carry out a solid phase carbonizingreaction.

When the graphitizable carbon source is, far example, a pitch, it ispreferred to cure a pitch component (so called anti-fusion treatment orstabilization) by gradually heating the pitch component at level of from150 to 350° C. in an oxidizing atmosphere such as in the atmosphere,whereby dehydrogenation, dehydration, oxidation and the like willproceed to form a three-dimension crosslinked structure.

The above cure treatment by forming e.g. the crosslinked structurementioned above may be carried out at the beginning of the carbonizingstep or at a completely different step from the carbonizing step, forexample, at a pretreatment step. Further, it is preferred to maintainthe graphitizable carbon source such as a pitch type material in anappropriate temperature condition at the beginning of the carbonizingstep, so that at least part of the graphitizable carbon source will beconverted to a mesophase pitch. Consequently, the activated carbonhaving large capacitance and relatively high bulk density is readilyobtained.

As described previously, when a phenol resin which is a thermosettingresin as the graphitizable carbon source is employed, both a resol resinand a novolac resin may be used. A resol resin may be used as it is. Inthe case of a novolac resin, it is preferred to add an excess of acuring agent such as formaldehyde, a resol resin orhexamethylenetetramine thereto and to heat and cure at from 150 to 350°C. for from 10 to 360 minutes.

When the mixture in a liquid state or in a thermally molten state iscured (anti-fusion treatment) in the manner described above, the mixturewill usually become a massive cured product. This cured product ispreferably crushed to, for example, several millimeters, beforesubjecting it to the carbonization, so that the resulting carbonizedproduct is easily ground.

When the cured product is ground, the grinding machine is notparticularly limited, and it is preferably the one capable of grindingthe cured product to at least several ten millimeters or less,preferably several millimeters or less, particularly preferably 1 mm orless. As such a grinding machine may be preferably employed the machinewhich has been previously mentioned. For example, a dodge crusher, asingle roll crusher, a double-roll crusher an edge runner, a jawcrusher, a cone crusher, a hammer mill, a desk crusher, a rod mill, aball mill, a tube mill, a roller mill, an attrition mill, a let mill, amicron mill, a micromizer, etc may be suitably employed.

The carbonization of the mixture (or the cured product) is carried outat a level of from 300 to to 2,000° C., preferably from 500 to 1,000°C., for from about 10 minutes to about 30 hours in a non-oxidizingatmosphere for example, an inactive gas such as nitrogen, argon, helium,xenon, neon, or a mixture thereof.

While an apparatus for carrying out the carbonization is notparticularly limited, a fixed bed heating furnace, a fluidized bedheating furnace, a moving bed heating furnace, an internal combustiontype or an outer heating type rotary kiln, an electric furnace, etc. issuitably employed.

(3) Finally, the above carbonized product is activated to an activatedcarbon which is a porous carbon material.

The activation is a process in which the porous structure of the solidcarbonized product obtained in the step of carbonization is grown anddeveloped to be a more fine porous structure. Basically, the activationmay be carried out by both a gas activation and a chemical activation.In order to adequately pull out characteristics of the activated carbonof the present invention, a chemical activation, especially an alkaliactivation is preferred, in which a mixture of alkali metal compound andthe carbonized product are mixed and subjected to heating and burningtreatment. As the alkali metal compound, an alkali carbonate such aspotassium carbonate or sodium carbonate may be used. However, it ispreferred to use at least one type of an alkali metal hydroxide such aspotassium hydroxide, sodium hydroxide, lithium hydroxide, rubidiumhydroxide or cesium hydroxide, and particularly preferred is potassiumhydroxide.

Such an alkali metal compound is added to and mixed with the carbonizedproduct in an amount of from 0.2 to 5.0 times the mass of the carbonizedproduct, and the mixture of the alkali metal compound and the carbonizedproduct is heated at a temperature of the melting point of the alkalimetal hydroxide or above, preferably from 300 to 1,000° C., morepreferably from 400 to 900° C., for from 30 minutes to 5 hours in anon-oxidizing atmosphere. At the above temperature, the alkali metalcompound erodes the carbon material strongly to emit carbon monoxide andcarbon dioxide, whereby a complexly developed porous structure will beformed.

The carbonized product after the alkali activation is preferablysubjected to a washing treatment, in which the carbonized product iswashed off with water to rinse the alkali component, and then,neutralized for example by an acid, and again, is washed with water torinse the acid. After the washing treatment, the carbonized product isadequately dried and sent to the next step.

A gas activation also can be employed in the activation process. In thegas activation, the carbonized product is heated at a temperature ofpreferably from 500 to 1,100° C., more preferably from 700 to 1,000° C.,for 5 minutes to 10 hours in a weakly oxidizing gas atmospherecontaining at least one of steam, carbon dioxide (combustion gas),oxygen, hydrogen chloride, chlorine, etc. Thus, unorganized part of thecarbonized product is contacted and reacted with the above gas, wherebysuch part will be decomposed and consumed to form a fine porousstructure.

The activation can be carried out by a combination of an alkaliactivation and a gas activation.

While an apparatus for carrying out the activation is not particularlylimited, an apparatus similar to the one which is used in thecarbonization step can be employed. For example, a fixed heatingfurnace, a fluidized bed heating furnace, a moving bed heating furnace,an internal combustion type or an outer heating type rotary kiln, or anelectric furnace, is suitably employed.

In the activation process, many fine pores having a diameter of at arevel of from 1 to 2 nm in the carbonized product, whereby activatedcarbon having a specific surface area from 500 to 2,800 m²/g isobtained.

Further, in the activation process, its conditions is preferablyselected so that a mass reduction rate of the carbonized product afterthe activation process is from 5 to 50 mass %, preferably from 10 to 40mass % in the alkali activation and from 30 to 90%, more preferably from50 to 80% in the gas activation, respectively. Consequently, thespecific surface area of the activated carbon obtained will be in thepreferred range mentioned above.

The carbonized product after the activation step and the washing stepdescribed above is preferably ground to form a fine product having anaverage particle diameter of from 0.5 to 30 μm, preferably from 1 to 10μm. A grinding machine used in this grinding process is not particularlylimited, a ball mill, a vibration mill, an attrition mill, etc. issuitably employed to obtain a powder having a particle diametermentioned above.

In the present invention, the stacking structure of the activated carbonis obtained by a reflection method using a X-ray diffraction apparatus:RU-3 type manufactured by Rigaku Denki Company Ltd.

Further, the specific surface area of the activated carbon is a valuemeasured by Autosorb-1 manufactured by Quantachrome Corporation ltd, oran apparatus having similar functions.

That is, a specific surface area is measured by using a BET multipointmethod within the range of from the relative pressure of from 0.001 to0.05 in an adsorption-isothermal curve, which is obtained by adsorbing anitrogen gas to a specimen at a temperature of liquid nitrogen. Thespecimen is dried at 200° C. for at least 12 hours under vacuum prior tothe above measurement.

The pore volume is obtained from the amount of a nitrogen gas at therelative pressure of around 0.99 in an adsorption-isothermal curvementioned above.

According to the present invention, an electric double layer capacitorcomprising an electrode mainly containing the activated carbon havingthe above fine pore characteristic as a electrode material, can beprovided. Preferably, the capacitor comprising an organic typeelectrolyte solution obtained by dissolving an electrolyte into anorganic solvent as an electrolyte solution can be provided.

As the electrolytic solution to be used for the capacitor of the presentinvention, an aqueous or an organic electrolytic solution may bebasically used. However, preferred is an organic electrolytic solutionsince the amount of energy accumulated per unit volume tends toincrease. In the case of using an organic electrolytic solution, thedecomposition voltage of an organic electrolytic solution is at leasttwice that of an aqueous electrolytic solution, and accordingly it isfavorable to use an organic electrolytic solution from the viewpoint ofthe energy density, which is proportional to half of the product of thecapacitance and the square of the voltage.

The electrode for an electric double layer capacitor of the presentinvention comprises, more specifically, the above carbon material and abinder, and preferably an electrical conductivity-imparting material.This electrode is formed, for example, in such a manner that a powder ofthe carbon material and a binder such as polytetrafluoroethylene andpreferably an electrically conductive material are adequately kneaded inthe presence of a solvent such as an alcohol, molded into a sheet form,followed by drying, and then bonding it to a current collector by meansof e.g. an electrically conductive adhesive. Otherwise, a powder of thecarbon material and a binder, and preferably, an electrically conductivematerial may be mixed with a solvent to obtain a slurry, which is thencoated on a current collector metal foil, followed by drying to obtainan electrode incorporated with the current collector.

As the binder, for example, polytetrafluoroethylene, polyvinylidenefluoride, a fluoroolefin/vinyl ether copolymerized crosslinked polymer,carboxymethyl cellulose, polyvinyl pyrrolidone, polyvinyl alcohol orpolyacrylic acid may be used. The content of the binder in the electrodeis preferably from about 0.5 to about 20 mass % based on the totalamount of the activated carbon and the binder. If the content of thebinder is less than 0.5 mass %, the strength of the electrode tends tobe insufficient, and if it exceeds 20 mass %, the electrical resistancetends to increase and the capacitance tends to decrease. The amount ofthe binder incorporated is more preferably from 0.5 to 10 mass %, fromthe viewpoint of the balance between the capacitance and the strength ofthe electrode. Here, as a crosslinking agent for the crosslinkedpolymer, an amine, a polyamine, a polyisocyanate, a bisphenol or aperoxide is preferred.

As the electrical conductivity-imparting material, a powder of carbonblack, natural graphite, artificial graphite, titanium oxide orruthenium oxide may be used. Among them, ketjen black or acetylene blackas one type of carbon black is preferably used since the effect toimprove electrical conductivity is significant with a small amount.

The amount of the electrical conductivity-imparting material such ascarbon black in the electrode is preferably at least 5 mass %,particularly preferably at least 10 mass %, based on the total amount ofthe activated carbon powders and the electrical conductivity-impartingmaterial. If the amount is too large, the proportion of the activatedcarbon incorporated will decrease, whereby the capacitance of theelectrode will decrease. Accordingly, the incorporated amount ispreferably less than 40 mass % or less, particularly preferably lessthan 30 mass % or less.

The solvent for forming a slurry is preferably one capable of dissolvingthe above binder, and e.g. N-methylpyrrolidone, dimethylformamide,toluene, xylene isophorone, methyl ethyl ketone, ethyl acetate, methylacetate, dimethyl phthalate, methanol, ethanol, isopropanol, butanol orwater may optionally be selected.

A current collector for the electrode may be any electrical conductor solong as it has electrochemical and chemical corrosion resistance. Forexample, stainless steel, aluminum, titanium, tantalum or nickel may beused. Among them, stainless steel and aluminum are preferred from theviewpoint of both performance and price.

The current collector may be in a form of a foil, may be a foam metal ofnickel or aluminum having a three-dimensional structure, or may be a netor a wool of stainless steel.

As the electrolytic solution to be used for an electric double layercapacitor of the present invention, a known aqueous or organicelectrolytic solution may be used. Among them, most preferable effectcan be obtained when an organic electrolytic solution is employed.

As the organic solvent, it is preferred to use at least one solventselected from the group consisting of electrochemically stable ethylenecarbonate, propylene carbonate, butylene carbonate, γ-butyrolactone,sulfolane, a sulfolane derivative, 3-methylsulfolane,1,2-dimethoxyethane, acetonitrile, glutaronitrile, valeronitrile,dimethylformamide, dimethylsulfoxide, tetrahydrofuran, dimethoxyethane,methylformate, dimethyl carbonate, diethyl carbonate and ethyl methylcarbonate. They may be used as a mixture.

As the electrolyte for the organic electrolytic solution, preferred is asalt comprising a quaternary onium cation represented by R¹R²R³R⁴N⁺ orR¹R²R³R⁴P⁺ (wherein each of R¹, R², R³ and R⁴ which are independent ofone another, is a C₁₋₆ alkyl group) and an anion selected from the groupconsisting of BF₄ ⁻, PF₆ ⁻, ClO₄ ⁻, CF₃SO₃ ⁻ and (SO₂R⁵)(SO₂R⁵)N⁻(wherein each of R⁵ and R⁶, which are independent of each other, is aC₁₋₄ alkyl group or alkylene group, and R⁵ and R⁶ may form a ring).Specific examples of the preferred electrolyte include (C₂H₅)₄NBF₄,(C₂H₅)₃(CH₃)NBF₄, (C₂H₅)₄PBF₄ and (C₂H₅)₃(CH₃)PBF₄. The concentration ofsuch a salt in the electrolytic solution is preferably from 0.1 to 2.5mol/l, more preferably from 0.5 to 2 mol/l.

As the separator to be interposed between the positive electrode and thenegative electrode, a non-woven fabric of polypropylene fiber or glassfiber, or synthetic cellulose may, for example, be suitably used.

The electric double layer capacitor of the present invention may haveany structure of a coin type wherein a pair of sheet electrodes with aseparator interposed therebetween is accommodated in a metal casingtogether with an electrolytic solution, a wound type wherein a pair ofpositive and negative electrodes is wound with a separator interposedtherebetween, and a laminate type wherein a plurality of sheetelectrodes are laminated with a separator interposed therebetween.

Different from the conventional activated carbon in which the carbonmaterial constituting its skeleton is only made of a highnon-graphitizability carbon component, or a high graphitizability carboncomponent, or a simple mixture of these two kinds of components, theactivated carbon of the present invention is considered to have astructure in which a high non-graphitizability carbon component and ahigh graphitizability carbon component are homogeneously mixed on theorder of micrometer or less, and are dispersed and strongly connectedwith each other. Accordingly, it is estimated that the skeleton ofexpandable and high graphitizability carbon component will be restrictedby the skeleton of the high non-graphitizability carbon component whichis small in the volume change at the time of charging and discharging.

From the above reason, according to the present invention, there isprovided an activated carbon for electric double layer capacitorelectrode, which has good characteristics of the two type activatedcarbon: A high capacitance, low deterioration of the performances aftera repeating charging-discharging cycle and a restricted expansion of theactivated carbon particles by ion adsorption which are attained by theactivated carbon derived from a high non-graphitizability carboncomponent, and large capacitance per unit volume caused by a high bulkdensity which are attained by the activated carbon derived from a highgraphitizability carbon component. At the same time, in the activatedcarbon of the present invention, the weaknesses of the two type carbonare compensated.

Now, the present invention will be explained in further detail withreference to Examples and Comparative Examples. However, it should beunderstood that the present invention is by no means restricted to suchspecific Examples.

EXAMPLE 1

(1) 50 parts by mass of a solid resol resin having a softening point of70° C. as a non-graphitizable carbon source and 50 parts by mass of apetroleum type pitch having a softening point of 150° C. as agraphitizable carbon source were weighted out. They were dissolved andmixed into 200 parts by mass of tetrahydrofuran (hereinafter referred toas THF) at 25° C., followed by reduced-pressure distillation using arotary evaporator to remove THF to obtain a homogeneous mixture of theresol resin and the pitch.

Then, the mixture was put in a crucible made of alumina. The mixture washeated to 300° C. at a heating rate of 30° C./hour in the atmosphere andkept at 300° C. for 2 hours to cure the mixture.

(2) The resulting cured mixture was ground using a Jaw crusher toparticle sizes of several millimeters or less, and again put in acrucible. The crucible was heated to 800° C. at a heating rate of 250°C./hour in an atmosphere of nitrogen and kept at 800° C. for 2 hours toobtain a carbonized product. The decrease in the mass during thecarbonization was 40%.

(3) 200 parts by mass of potassium hydroxide (hereinafter referred to asKOH) was mixed with 100 parts by mass of the carbonized product obtainedabove, and then the mixture was put in a crucible made of silver,followed by heating in an atmosphere of nitrogen at a heating rate of250° C./hour to 800° C. and kept at 800° C. for 2 hours to carry out anactivation treatment. The activated product was washed adequately withde-ionized water, neutralized with 0.1 N-hydrochloric acid, and furtherthoroughly washed with de-ionized water. The washed activated productwas dried under vacuum at 250° C. and was pulverized using a ball millmade of alumina to an average particle diameter of 5 μm.

The activated carbon thus obtained had a stacking structure having 2layers or less in proportion of 39%, the stacking structure having 5layers or more in a proportion of 19%, a specific surface area of 1,900m²/g, and a total pore volume of 0.9 cm³/g.

(4) Then, a mixture consisting of 80 mass % of the obtained an activatedcarbon, 10 mass % of Furnace-Black (manufactured by KETJENBLACKInternational Company, trade name: KETJENBLACK EC) as an electricalconductivity-imparting material and 10 mass % of apolytetrafluoroethylene as a binder, was kneaded while adding ethanolthereto. Then, the mixture was rolled to obtain an electrode sheethaving a thickness of 0.65 mm, which was dried at 200° C. for 2 hours.Then, two electrodes having a diameter of 12 mm were punched from thesheet, and those two electrodes as a positive electrode and a negativeelectrode, were respectively bonded to an aluminum sheet as a currentcollector by means of a graphite type electrically conductive adhesive.The mass and size of each above punched electrode were measured in thedried state before it is bonded to the aluminum sheet.

Then, the positive and the negative electrodes bonded to the aluminumsheet were dried under vacuum at 250° C. for 4 hours, and impregnatedwith a propylene carbonate solution containing (C₂H₅)₃(CH₃)NBF₄ at aconcentration of 1 mil/l in an atmosphere of dry argon. The twoelectrodes were disposed so that they faced each other by means of aseparator made of polypropylene non-woven fabric, and the electrodeswere sandwiched between two glass sheets and constricted under aconstant pressure and then fixed. Then, a coin-type capacitor cell wasconstructed so that the above fixed electrodes were completely immersedin a propylene carbonate solution containing (C₂H₅)₃(CH₃)NBF₄ at aconcentration of 1 mol/l.

A voltage of 2.5 V was applied to the accomplished coin-type capacitorcell, and the capacitance and the internal resistance were measured,whereupon they were 4.5 F and 8.2 Ω/cm² respectively.

Further, the coin-type capacitor cell which bad been used in themeasurement was disassembled in its charged state, and each thickness ofthe positive and the negative electrodes was measured. Then, theexpansion coefficient of the electrodes after charging was obtained bythe ratio to the thickness of the electrode before the impregnation withthe electrolyte. As a result, an average expansion coefficient of thepositive and the negative electrodes was 15%.

EXAMPLE 2

(1) 5 parts by mass of a solid resol resin having a softening point of70° C. as a non-graphitizable carbon source and 50 parts by mass of apetroleum type pitch having a softening point of 90° C. as agraphitizable carbon source were weighted out. They were heated to 150°C. and melt-mixed with each other, and cooled to be in a solid form. Theresulting solid mixture was ground using a jaw crusher to particle sizesof several millimeters or less. Then, hexamethylenetetramine was addedto the ground mixture in an amount of 10 mass % of the resol resincontained in the mixture, and then was ground and mixed using a ballmill to an average particle diameter of 70 μm. Then, the mixed powderthus obtained was heated to 200° C. at a heating rate of 200° C./hour inthe atmosphere and kept at 200° C. for 2 hours to obtain a curedproduct.

(2) The cured product was put in a crucible made of alumina and washeated to 800° C. at a heating rate of 250° C./hour in an atmosphere ofnitrogen and kept at 800° C. for 2 hours to obtain a carbonized product.

(3) Similarly to the procedure as in Example 1, the obtained carbonizedproduct was activated using KOH, washed, dried, and pulverized to obtainactivated carbon having an average particle diameter of 5 μm. Thisactivated carbon had a stacking structure having 2 layers or less in aproportion of 52%, the stacking structure having 5 layers or more in aproportion of 14%, a specific surface area of 2,000 m²/g, and a totalpore volume of 1.0 cm³/g.

(4) Using the activated carbon obtained above, a coin-type electricdouble layer capacitor was assembled in the same manner as in Example 1.A voltage of 2.5 V was applied to the capacitor, and the capacitance andthe internal resistance were measured, whereupon they were 5.1 F and 7.5Ω/cm², respectively.

Then, the coin-type capacitor cell which had been used in themeasurement was disassembled in its charged state, and each thickness ofthe positive and the negative electrodes was measured. Then, theexpansion coefficient of the electrodes after charging was obtained bythe ratio to the thickness of the electrode before the impregnation withthe electrolyte. As a result, an average expansion coefficient of thepositive and the negative electrodes was 12%.

EXAMPLE 3

(1) 80 parts by mass of a liquid resol resin (solid parts; 50%) obtainedby dissolving de-hydrated resol resin into ethylene glycol as anon-graphitizable carbon source and 20 parts by mass of a petroleum typepitch which was ground to an average particle diameter of 5 μm and had asoftening point of 250° C. as a graphitizable carbon source wereweighted out. They were mixed to be in a slurry state, and again mixedhomogeneously using a feather shaped stirrer. The resulting mixture washeated to 150° C., melt-mixed with each other, and cooled to be in asolid form. Similarly to the procedure as in Example 1, the obtainedmixture was cured, carbonized, and activated to obtain activated carbonhaving a specific surface area of 2,2200 m²/g, and a total pore volumeof 1.10 cm³/g. This activated carbon had a stacking structure having 2layers or less in a proportion of 60%, a stacking structure having 5layers or more in a proportion of 10%.

(2) Using the activated carbon obtained above, a coin-type electricdouble layer capacitor was assembled in the same manner as in Example 1except that (C₂H₅)₄NBF₄ was used instead of (C₂H₅)₃(CH₃)NBF₄. A voltageof 2.5 V was applied to the capacitor, and the capacitance and theinternal resistance were measured, whereupon they were 5.5 F and 7.1Ω/cm², respectively.

Then, the coin-type capacitor cell which had been used in themeasurement was disassembled in its charged state, and each thickness ofthe positive and the negative electrodes was measured. Then, theexpansion coefficient of the electrodes after charging was obtained bythe ratio to the thickness of the electrode before the impregnation withthe electrolyte. As a result, an average expansion coefficient of thepositive and the negative electrodes was 10%.

EXAMPLE 4

(1) 20 parts by mass of a urea resin as a non-graphitizable carbonsource and 80 parts by mass of a vinyl chloride resin as a graphitizablecarbon source were weighted out. They were dissolved and mixed into 200parts by mass of THF at 25° C. with each other, followed byreduced-pressure distillation using a rotary evaporator to remove THF toobtain a homogeneous mixture of the urea resin and the vinyl chlorideresin.

The resulting mixture was ground using a jaw crusher to a particle sizeof several millimeters or less, and was heated to 180° C. at a heatingrate of 100° C./hour in an atmosphere of nitrogen and kept for 30minutes to cure the urea resin component.

Further, the resulting product was heated to 400° C. at a heating rateof 100° C./hour to thermally decompose the vinyl chloride resincomponent to pitch and kept at 400° C. for 2 hours to convert the pitchcomponent to its mesophase pitch.

(2) The resulting mixture was ground using a ball mill to an averageparticle diameter of 100 μm, and was subjected to non-molten treatmentin a small rotary kiln at 250° C. for 3 hours in the atmosphere, andthen was heated to 800° C. at a heating rate of 250° C./hour in anatmosphere of nitrogen and kept at 800° C. for 2 hours to obtain acarbonized product.

(3) Similarly to the procedure as in Example 1, the obtained mixture wasactivated, washed, and pulverized to obtain an activated carbon. Thisactivated carbon had a stacking structure having 2 layers or less in aproportion of 55%, a stacking structure having 5 layers or more in aproportion of 12%, 1,700 m²/g of the specific surface area, and 0.75cm³/g of the total pore volume.

(4) Using the activated carbon obtained above, a coin-type electricdouble layer capacitor was assembled in the exactly same manner as inExample 1. A voltage of 2.5 V was applied to the capacitor, and thecapacitance and the internal resistance were measured, whereupon theywere 4.6 F and 8.5 Ω/cm², respectively.

Then, the coin-type capacitor cell which had been used in themeasurement was disassembled in its charged state, and each thickness ofthe positive and the negative electrodes was measured. Then, theexpansion coefficient of the electrode after charging was obtained bythe ratio to the thickness of the electrode before the impregnation withthe electrolyte. As a result, an average expansion coefficient of thepositive and the negative electrodes was 18%.

EXAMPLE 5

(1) 1 mol of formaldehyde was mixed with 0.8 mol of a phenol resin and0.005 mol of sodium hydroxide was added thereto as a catalyst. Then, 40mass % of a coal tar pitch having a softening point of 110° C. was addedto the mixture of formaldehyde and phenol resin, and was refluxed at 95°C. for 1.5 hour, followed by gradually reducing the pressure to about6.7 kPa. Then the mixture was heated to the internal temperature of 120°C., and retuned to the room temperature to obtain a mixture of theliquid phenol resin and the pitch. To this mixture containing the liquidphenol resin was added hexamethylenetetramine in an amount of 10 mass %,and cured in the same condition as in Example 1.

(2) The resulting carbonized product was ground using a Joe crusher to aparticle size of several millimeters or less, and was kept at 700° C.for 2 hours in a crucible made of alumina to obtain a carbonizedproduct. The decrease in the mass during the carbonization was 45%.

(3) The resulting was treated in the same manner as in Example 1 toobtain an activated carbon. This activated carbon had a stackingstructure having 2 layers or less in a proportion of 55%, a stackingstructure having 5 layers or more in a proportion of 11%, a specificsurface area of 2,100 m²/g, and a the total pore volume of 1.05 cm³/g.

(4) using the activated carbon obtained above, electrodes sheet wereprepared, and then a coin-type electric double layer capacitor wasassembled in the same manner as in Example 1.

The capacitance and the internal resistance of the accomplishedcoin-type electric double layer capacitor were measured in the samemanner as in Example 1, whereupon they were 5.3 F and 7.3 Ω/cm²,respectively.

Then, the expansion coefficient of the electrodes was measured in thesame manner as in Example 1. An average expansion coefficient of thepositive and the negative electrodes was 11%.

EXAMPLE 6

A petroleum pitch having softening point of 305° C. was ground using aball mill to an average particle diameter of 10 μm and then was heatedin an oven and in the atmosphere at a heating rate of 60° C./hour to320° C. and kept for 2 hours so as to carry out its non-moltentreatment. 50 parts by mass of the powder obtained above as agraphitizable carbon source and 50 parts by mass of a solid resol resinpowder having an average particle diameter of 30 μm and a softeningpoint of 70° C. as a non-graphitizable carbon were weighted out. To thismixture was added hexamethylenetetramine in an amount of 10 mass % ofthe resol resin and mixed with each other homogeneously using V-typeblender.

The obtained mixture powder was heated to 200° C. at a heating rate of200° C./hour in the atmosphere and kept for 2 hours to obtain a curedproduct. This cured product was carbonized, activated, washed, andground in the same manner as in Example 1 to obtain an activated carbonhaving a specific surface area of 1,900 m²/g and a total pore volume of0.9 cm³/g. This activated carbon had a stacking structure having 2layers or less in a proportion of 40%, a stacking structure having 5layers or more in a proportion of 20%.

Using the activated carbon obtained above, a coin-type electric doublelayer capacitor was assembled in the same manner as in Example 1. Byapplying a voltage of 2.5 V to the capacitor, the capacitance and theinternal resistance were measured. They were 4.1 F and 8.0 Ω/cm²,respectively.

Further, the coin-type capacitor cell which had been used in themeasurement was disassembled in its charged state, and each thickness ofthe positive and the negative electrodes was measured. Then, theexpansion coefficient of the electrodes after charging was obtained bythe ratio to the thickness of the electrode before the impregnation withthe electrolyte. As a result, an average expansion coefficient of thepositive and the negative electrodes was 18%.

Comparative Example 1

(1) To a novolac resin powder having a melting point of 100° C. as anon-graphitizable carbon source was added hexamethylenetetramine in anamount of 10 mass % of the resin, and was adequately mixed with eachother. The resulting mixture was heated to 200° C. at a heating rate of200° C./hour in the atmosphere and kept at 200° C. for 2 hours to obtaina cured product.

The obtained cured product was carbonized, activated, washed, and groundin the exactly same manner as in Example 1 to obtain an activatedcarbon. This activated carbon had a stacking structure having 2 layersor less in a proportion of 86%, but a stacking structure having 5 layersor more was not recognized. The specific surface was 2,300 m²/g and thetotal pore volume was 1.2 cm³/g.

(2) Using the activated carbon obtained above, a coin-type electricdouble layer capacitor was assembled in the same manner as in Example 1.By applying a voltage of 2.5 V to the capacitor, the capacitance and theinternal resistance were measured. They were 3.00 F and 7.0 Ω/cm²,respectively.

Further, the coin-type capacitor cell which had been used in themeasurement was disassembled in its charged state, and each thickness ofthe positive and the negative electrodes was measured. Then, expansioncoefficient of the electrodes after charging was obtained by the ratioto the thickness of the electrode before the impregnation with theelectrolyte. As a result, an average expansion coefficient of thepositive and the negative electrodes was 9%.

Comparative Example 2

(1) A petroleum pitch having softening point of 305° C. as agraphitizable carbon source was ground using a ball mill to an averageparticle diameter of 200 μm and then was heated in a rotary kiln and inthe atmosphere at a heating rate of 60° C./hour to 350° C. and kept for5 minutes so as to carry out its anti-fusion treatment.

The resulting non-melted treated powder was heated to 800° C. at aheating rate of 250° C./hour in an atmosphere of nitrogen and kept for 2hours to obtain a carbonized product.

The obtained carbonized product was activated, washed, and ground in thesame manner as in Example 1 to obtain activated carbon. This activatedcarbon had a stacking structure having 2 layers or less in a proportionof 54% and a stacking structure having 5 layers or more in a proportionof 11%. The specific surface area was 1,650 m²/g and the total porevolume was 0.65 cm³/g.

(2) Using the activated carbon obtained above, a coin-type electricdouble layer capacitor was assembled in the same manner as in Example 1.By applying a voltage of 2.5 V to the capacitor, the capacitance and theinternal resistance were measured. They were 4.5 F and 9.0 Ω/cm²,respectively.

Further, the coin-type capacitor cell which had been used in themeasurement was disassembled in its charged state, and each thickness ofpositive and negative electrodes was measured. Then, expansioncoefficient of the electrode after charging was obtained by the ratio tothe thickness of the electrode before the impregnation with theelectrolyte. As a result, an average expansion coefficient of positiveand negative electrodes was 32%.

The above results are summarized in Table 2 and Table 3.

TABLE 2 Mixing ratio of Amount of the Amount of the non-graphitizablestacking stacking carbon source/a structure structure graphitizablehaving 2 layers having 5 layers carbon source or less (%) or less (%)Ex. 1 1:1 39 19 Ex. 2 1:1 52 14 Ex. 3 2:1 60 10 Ex. 4 1:1 55 12 Ex. 51:1 55 11 Ex. 6 1:1 40 20 Comp. 1:0 86 0 Ex. 1 Comp. 0:1 21 22 Ex. 2

TABLE 3 Specific Total Electrode surface pore expansion area volumeCapacitance Resistance coefficient (m²/g) (cm³/g) (F) (Ω/cm²) (%) Ex. 11,900 0.90 4.5 8.2 15 Ex. 2 2,000 1.00 5.1 7.5 12 Ex. 3 2,200 1.10 5.57.1 10 Ex. 4 1,700 0.75 4.6 8.5 18 Ex. 5 2,100 1.05 5.3 7.3 11 Ex. 61,900 0.90 4.1 8.0 18 Comp. 2,300 1.20 3.0 7.0  9 Ex. 1 Comp. 1,650 0.654.5 9.0 32 Ex. 2

The activated carbon of the present invention has a mixed structure ahigh graphitizability part and a high non-graphitizability part, whereinthe skeleton of a high non-graphitizability activated carbon having ahigh mechanical strength and an low volume change at the time ofcharging and discharging attains apparently high capacitance per unitvolume and at the same time restricts the skeleton of a highgraphitizability activated carbon having a large volume expansion at thetime of charging.

Accordingly, the volume expansion of the activated carbon at the time ofcharging and discharging can be restricted so that an electrode materialfor an electric double layer capacitor and an electric double layercapacitor which have large capacitance per volume and excellentreliability for long time of period can be provided.

The entire disclosure of Japanese Patent Application No. 2000-192393filed on Jun. 27, 2000 including specification, claims and summary areincorporated herein by reference in its entirety.

What is claimed is:
 1. A process for producing an activated carbon foran electric double layer capacitor electrode, which comprises: (1) astep of mixing a carbon source which forms a non-graphitizable carbon byheating and a carbon source which forms a graphitizable carbon byheating; (2) a step of carbonizing the resulting mixture by means ofheating; and (3) a step of activating the carbonized material, theobtained an activated carbon comprising a stacking structure having atmost 2 layers in a proportion of from 25 to 80% and a stacking structurehaving at least 5 layers in a proportion of from 2 to 30% in thedistribution of a stacking structure as obtained by analysis of theX-ray diffraction pattern of (002) plane, and having a specific surfacearea of from 500 to 2,800 m²/g and having a total pore volume of from0.5 to 1.8 cm³/g.
 2. The process for producing an activated carbonaccording to claim 1, wherein the carbon source to form anon-graphitizable carbon is a thermosetting resin, and the carbon sourceto form graphitizable carbon is a pitch.
 3. The process for producing anactivated carbon according to claim 1, wherein the carbon source to formnon-graphitizable carbon and the carbon source to form graphitizablecarbon are mixed in a mass ratio of the former/the latter being from9.5/0.5 to 1/9.
 4. The process for producing an activated carbonaccording to claim 1, wherein in the step of carbonizing the carbonsource to form a graphitizable carbon, the carbonization is carried outat least a part of the carbon source has been converted to a mesophasepitch.
 5. The process for producing an activated carbon according toclaim 4, wherein the carbon source to form a non-graphitizable carbon isa thermosetting resin, and the carbon source to form a graphitizablecarbon is a pitch.
 6. The process for producing an activated carbonaccording to claim 1, wherein the an activating step is carried out byheating an alkali metal compound and the carbonized product in anon-oxidizing atmosphere.
 7. The process for producing an activatedcarbon according to claim 1, wherein the carbon source to form anon-graphitizable carbon is a thermosetting resin, and the carbon sourceto form a graphitizable carbon is pitch.
 8. The process for producing anactivated carbon according to claim 1, wherein during at least from thestep of mixing to the step of carbonization, at least one of the carbonsource to form a non-graphitizable carbon and the carbon source to forma graphitizable carbon is in a liquid state.
 9. The process forproducing an activated carbon according to claim 8, wherein the carbonsource to form a non-graphitizable carbon is a thermosetting resin, andthe carbon source to form a graphitizable carbon is a pitch.
 10. Aprocess for producing electric double layer capacitor comprisingelectrodes containing an activated carbon and an organic typeelectrolytic solution, said activated carbon being produced by (1) astep of mixing a carbon source which forms a non-graphitizable carbon byheating and a carbon source which forms a graphitizable carbon byheating; (2) a step of carbonizing the resulting mixture lay means ofheating; and (3) a step of activating the carbonized material, and theactivated carbon comprising a stacking structure having 2 layers or lessin a proportion of from 25 to 80% and a stacking structure having 5layers or more in a proportion of from 2 to 30% in the distribution of astacking structure as obtained by analysis of the X-ray diffractionpattern of (002) plane, and having a specific surface area of from 500to 2,800 m²/g and a total pore volume of from 0.5 to 1.8 cm³/g.
 11. Theprocess for producing an electric double layer capacitor according toclaim 10, wherein the carbon source to form a non-graphitizable carbonis a thermosetting resin, and the carbon source to form a graphitizablecarbon is a pitch.
 12. The process for producing an electric doublelayer capacitor according to claim 10, wherein the carbon source to forma non-graphitizable carbon and the carbon source to form a graphitizablecarbon are mixed in a mass ratio of former/latter being from 9.5/0.5 to1/9.
 13. The process for producing an electric double layer capacitoraccording to claim 10, wherein in the step of carbonization, the carbonsource to form a graphitizable carbon is carbonized after at least ofwhich has been converted to a mesophase pitch.
 14. The process forproducing an electric double layer capacitor according to claim 13,wherein the carbon source to form a non-graphitizable carbon is athermosetting resin, and the carbon source to form a graphitizablecarbon is a pitch.
 15. The process for producing an electric doublelayer capacitor according to claim 10, wherein the activating step iscarried out by heating an alkali metal compound and the carbonizedmaterial in a non-oxidizing atmosphere.
 16. The process for producing anelectric double layer capacitor according to claim 10, wherein at leastfrom the step of mixing to the step of carbonization, at least one ofthe carbon source to form a non-graphitizable carbon and the carbonsource to form a graphitizable carbon is in a liquid state.